>>6254899I'd assume just because he's the main character and the first Guyver figure in a long time. Guyver has a small, but very dedicated fanbase, so I'm sure they were quick to get on top of that. He might be discontinued as well.
I think Guyver III is the more popular of the two protagonist Guyvers though, so he might face similar, if not worse, markups. Compare to the two IIF figures, which represent a relatively new character in the manga and a character that only appeared in one movie.